The present invention relates to a scanning optical system that scans a light beam on an object surface by deflecting the light beam with a rotating polygon mirror and passing the deflected light beam through an image forming optical system that converges the light beam on the object surface.
The scanning optical system is applied, for example, to apparatuses such as laser printers, digital copy machines, and laser facsimile machines, which form images in accordance with an electrophotographic imaging process.
In the scanning optical system, a laser beam modulated in accordance with image data is deflected by a rotating polygon mirror. The deflected laser beam is then converged on a photo-sensitive surface by an image forming optical system so as to form a beam spot that scans on the photo-sensitive surface in a main scanning direction. By repeatedly scanning the modulated laser beam while moving the photo-sensitive surface in a direction perpendicular to the main scanning direction, or an auxiliary scanning direction, in an appropriate manner, a two dimensional image made up of a plurality of dots can be formed on the photo-sensitive surface.
A critical issue in designing the scanning optical system is how to remove ghost light generated by unwanted reflection of the laser beam at one or more surfaces of the optical elements constituting the scanning optical system.
Ghost light generated by a lens of the image forming optical system, which is disposed between the polygon mirror and the photo-sensitive surface, may travel toward the polygon mirror, and then be reflected again toward the image forming optical system to strike the photo-sensitive surface.
In the above-mentioned case, if the ghost light is reflected by the same reflection surface of the polygon mirror that is reflecting the laser beam, the ghost light scans on the photo-sensitive surface at the same speed as the laser beam. Since the energy of the ghost light is much lower than that of the laser beam, the ghost light in this case does not expose the photo-sensitive surface and form a ghost image thereon.
On the contrary, if the ghost image is incident on the reflection surface of the polygon mirror adjacent to that reflecting the laser beam, the ghost light may move on the photo-sensitive surface at a much slower speed or may even substantially stop at a particular location, depending on the shape of the lens generating the ghost light. In such a case, the photo-sensitive surface is exposed and a ghost image appears.
The effect of the above-mentioned ghost light is critical in a single polygon mirror multi-beam scanning optical system in which a plurality of laser beams are simultaneously scanned by a single polygon mirror on respective photoconductive drums, each corresponding to a different color, to produce a-color image.
In the single polygon mirror multi-beam scanning optical system, the plurality of (even number of) laser beams are obliquely incident on the reflection surface of the polygon mirror from both sides of a main scanning plane so that the light beams are distributed symmetrically with respect to a main scanning plane. Here, the main scanning plane is defined as a plane perpendicular to a rotation angle of the polygon mirror and intersecting each reflection surfaces of the polygon mirror at the center thereof.
The laser beams reflected by the polygon mirror spread in a fan shape from a substantially single point on the reflection surface. The reflected laser beams are again distributed symmetrically with respect to the main scanning plane. The reflected laser beams pass through respective image forming lens systems and strike respective photoconductive drums. The laser beams form latent images on the photoconductive drums which are subsequently developed with toners of different colors. The developed images are then transferred onto a single print sheet to form a color image.
In the single polygon mirror multi-beam scanning optical system, the ghost light generated by the image forming optical system may travel toward the polygon mirror on one side of the main scanning plane and be reflected to travel on the other side of the main scanning plane. If such a ghost light travel toward the polygon mirror along a plane that is parallel to the main scanning direction and includes the original laser beam generating the ghost light, the ghost light reflected by the polygon mirror becomes to travel within an optical path of one of the laser beams traveling on the other side of the main scanning plane since the laser beams are distributed symmetrically with respect to the main scanning plane. As a result, the ghost light impinges onto a photoconductive drum corresponding to a color different from that of the original laser beam generating the ghost light. Such a ghost image may be formed on each photoconductive drum, and thereby cause significant deterioration in the quality of the finally produced color image.
Therefore, there is a need for a scanning optical system that is capable of preventing the above-mentioned ghost light, which is reflected by the reflection surface of the polygon mirror adjacent to the reflection surface deflecting the light beam generating the ghost light, from entering again the image forming optical system.